/Engine/Source/Runtime/UMG/Public/Blueprint/GameViewportSubsystem.h
Adds the widget to the game's viewport in the section dedicated to the player. This is valuable in a split screen game where you need to only show a widget over a player's portion of the viewport.
bool AddWidgetForPlayer ( [UWidget](API\Runtime\UMG\Components\UWidget) * Widget, [ULocalPlayer](API\Runtime\Engine\Engine\ULocalPlayer) * Player, [FGameViewportWidgetSlot](API\Runtime\UMG\Blueprint\FGameViewportWidgetSlot) Slot )